Extensions - Revise prose for status check

[totten]

Overview

This is a follow-up to revise the prose introduced by #22464 (merged on the same 5.47.alpha1).

cc @colemanw @demeritcowboy

Before

Example with one missing dependency - which is available but not active. (This PR addresses the message in the middle; other messages are shown for context.)

Example with two missing dependencies - one available-inactive and one totally unknown:

Notes:

• Text containing the extension labels and keys is non-distinct. Assumes "label" is title-case.
• Uses <ul> without any intro text, so it it looks different from other messages.
• The title is singular ("Error") but may list multiple errors.

After

Example with one missing dependency - which is available but not active. (This PR addresses the message in the middle; other messages are shown for context.)

Example with two missing dependencies - one available-inactive and one totally unknown:

[civibot]

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